Phenylpropanoids, particularly flavonoids have been recently suggested as playing primary antioxidant functions in the responses of plants to a wide range of abiotic stresses. Furthermore, flavonoids are effective endogenous regulators of auxin movement, thus behaving as developmental regulators. Flavonoids are capable of controlling the development of individual organs and the whole-plant; and, hence, to contribute to stress-induced morphogenic responses of plants. The significance of flavonoids as scavengers of reactive oxygen species (ROS) in humans has been recently questioned, based on the observation that the flavonoid concentration in plasma and most tissues is too low to effectively reduce ROS. Instead, flavonoids may play key roles as signaling molecules in mammals, through their ability to interact with a wide range of protein kinases, including mitogen-activated protein kinases (MAPK), that supersede key steps of cell growth and differentiation. Here we discuss about the relative significance of flavonoids as reducing agents and signaling molecules in plants and humans. We show that structural features conferring ROS-scavenger ability to flavonoids are also required to effectively control developmental processes in eukaryotic cells.

There is a growing body of evidence that flavonoids do not primarily function as UV-B screening pigments in photoprotection. Recent findings support the idea that excess light stress, irrespective of the relative proportions of the solar wavebands reaching the leaf surface, up-regulates the biosynthesis of dihydroxy B-ring-substituted flavonoid glycosides, as a consequence of and aimed at countering the generation of ROS. Intriguingly, the very conditions that lead to the inactivation of antioxidant enzymes can also up-regulate the biosynthesis of antioxidant flavonoids, which suggests flavonoids constituting a secondary ROS-scavenging system in plants exposed to severe/prolonged stress conditions. H2O2 may diffuse out of the chloroplast at considerable rates and be transported to the vacuole, the storing site for flavonoids, by tonoplast intrinsic proteins, under severe excess light conditions. We suggest that the unanticipated key role of the vacuole in the ROS homeostasis might be mediated by flavonoids.

Whether flavonoids play significant antioxidant roles in plants challenged by photooxidative stress of different origin has been largely debated over the last few decades. A critical review of the pertinent literature and our experimentation as well, based on a free-of-scale approach, support an important antioxidant function served by flavonoids in plants exposed to a wide range of environmental stressors, the significance of which increases with the severity of stress. On the other side, some questions need conclusive answers when the putative antioxidant functions of plant flavonoids are examined at the level of both the whole-cell and cellular organelles. This partly depends upon a conclusive, robust, and unbiased definition of “a plant antioxidant”, which is still missing, and the need of considering the subcellular re-organization that occurs in plant cells in response to severe stress conditions. This likely makes our deterministic-based approach unsuitable to unveil the relevance of flavonoids as antioxidants in extremely complex biological systems, such as a plant cell exposed to an ever-changing stressful environment. This still poses open questions about how to measure the occurred antioxidant action of flavonoids. Our reasoning also evidences the need of contemporarily evaluating the changes in key primary and secondary components of the antioxidant defense network imposed by stress events of increasing severity to properly estimate the relevance of the antioxidant functions of flavonoids in an in planta situation. In turn, this calls for an in-depth analysis of the sub-cellular distribution of primary and secondary antioxidants to solve this still intricate matter.

It is known that the urban environment amplifies the effects of climate change, sometimes with disastrous consequences that put people at risk. These aspects can be affected by urban vegetation and planting design but, while there are thousands of papers related to the effects of climate change, a relatively limited number of them are directly aimed at investigating the role of vegetation as a mitigating factor in the urban context. This paper focuses on reviewing the research on the role of urban vegetation in alleviating the adverse conditions of the urban environment in order to provide some practical guidelines to be applied by city planners. Through an analysis of the documents found in Scopus, Web of Science, and Google Scholar using urban vegetation and climate change-related keywords we selected five major issues related to the urban environment: (1) particulate matter, (2) gaseous pollution, (3) noise pollution, (4) water runoff, (5) urban heat island effect. The analysis of existing knowledge reported here indicates that the roles of urban vegetation on the adverse effect of climate change could not be simply deemed positive or negative, because the role of urban green is also strongly linked to the structure, composition, and distribution of vegetation, as well as to the criteria used for management. Therefore, it could help to better understand the roles of urban green as a complex system and provide the foundation for future studies.